Resistance switching network and motor control system



Dec. 11, 1962 l. w. LICHTENFELS ETAL 3,068,390

RESISTANCE SWITCHING NETWORK AND MOTOR CONTROL SYSTEM Original FiledNov. 16, 1954 ZSheets-Sheet 1 6 6 5/ H F/ ls FFE /4 I I l *9 7 1 W5 1 Z3/ (E a r?" 1 r I N0 TCH Ol/MS 7 [)7 venaars: Char/es 6 Moon, [ra MLia/7 tc nf'c/s, by MW e/L" Attorney Dec. 11, 1962 1. w. LICHTENFELSEIAL 3, 68,390

RESISTANCE SWITCHING NETWORK AND MOTOR CONTROL SYSTEM Original FiledNov. 16, 1954 2 Sheets-Sheet 2 IL |1 [27 van tars.- A; Char/e5 6? Moon,[ra 14/ Lia/7 enfe/s,

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United States Patent Ofiice 3,068,390 Patented Dec. 11, 1962 3,0,390RESISTANCE SWITCHING NETWQRK AND MOTQR CQNTROL SYSTEM Ira W. Lichtenfelsand Charles G. Moon, Erie, Pa., as-

signors to General Electric Company, a corporation of New YorkContinuation of abandoned application Ser. No. 469,184, Nov. 16, 1954.This application June 24, 1958, Ser. No. 744,892

7 Claims. (Cl. 318-476) This invention relates to control resistors forelectric motors having series, series and shunt, or shunt field windingsof the kind in which, during acceleration and dynamic braking, theresistance is progressively cut out of the motor circuit to control thecurrent there-through and, more particularly, for resistance shunting ofmotor field windings of traction motors.

This application is a continuation of the original application, SerialNumber 469,184, filed November 16, 1945, by Ira W. Lichtenfels andCharles G. Moon, now abandoned, and assigned to the assignee of thisinvention.

It is well known that a D.C. type dynamoelectric machine produces verylittle voltage or when it is rotating well below its rated speed.Therefore, when this dynamoelectric machine is used as a motor, avariable controlled impedance in series with this varying motorimpedance is necessary to regulate the total impedance of the circuitand prevent excessive currents during acceleration under load.Similarly, during dynamic braking a variable regulated impedance isrequired to control the current.

In the art relating to the control of electric motors, requiringresistance switching systems, particularly but not exclusively tractionmotors, there has been a continual effort to provide a maximum number ofeffective resistance reduction steps with a minimum of resistor mass,resistor sections, and number of associated switching devices. Suchsystems comprise a plurality of resistor sections and correspondingswitches such as manual, electropneumatic, electromagnetic, orcam-operated contactors, whereby sections of a resistance may be shuntedin turn or placed in various connections of series of parallel withother sections to reduce the series impedance of the system foraccelerating during the motoring operation or decelcrating during thedynamic braking operation.

It is desirable to remove the resistance in a large number of steps ornotches, since, with the motor operating at a specific torque in any onenotch, movement of the controller to the next notch will cause anincrease in motor torque, and in order to obtain a maximum overallusable accelerationwithout wheel slippage or excessive currents, thetorque should be maintained as closely as practicable to a constantvalue. Consequently, it is desirable that the voltage increment or theresistance decrease shall be as small as practicable. On the other hand,the number of steps or notches available will generally be limited bythe number of resistor sections, contractors, or mass of resistancewhich can be accommodated considering space and the complexity and costof the control gear.

If it were practicable to use all of the resistor sections all of thetime in either series or parallel connection, this would greatly reducethe resistor mass necessary for a particular application. However,theoretical attempts to do this in the past have resulted in such anincrease in the number of switching contacts necessary that the mass ofthe switching mechanism becomes so great as to render the systemimpractical and unusable.

If it were practicable to use each of the switches for every notch, thiswould greatly reduce the number of switches necessary for a particularapplication. However, theoretical attempts to do this in the past haveresulted in such an increase in the number and mass of resistor sectionsnecessary the system is impractical and unusuable.

Every variable resistance system is use is an attempt to attain theultimate in these two ideals; minimum resistor mass and taps, andminimum number of switching devices.

Therefore, an object of this invention is to provide a multistepelectrical resistance switching network having an increased number ofresistance steps for the number of contactors used, or for the samenumber of resistance steps to reduce the number of contactors used.

A further object is to provide an electrical resistance switchingnetwork which utilizes each portion of the resistor a greater percentageof the time.

A still further object is to provide an electrical resistance switchingnetwork which lends itself to simplified analysis.

Another object is to provide an electrical resistance switching networkin which the resistance values of the steps are in geometricprogression.

Another object is to provide an electrical resistance switching networkin which switches close but do not open current carrying circuits, thusreducing contact wear and eliminating arc blow out attachments.

Another object is to provide an electrical resistance switching networkwhich can conveniently be set up and is practical for any number ofresistance steps due to the symmetry of the circuit arrangement.

Another object is to provide a motor control system using this improvedresistance switching network.

Further objects and advantages of this invention Will become apparentand this invention will be better understood from the followingescription, referring to the accompanying drawings. The features ofnovelty which characterize this invention will be pointed out withparticularity in the claims annexed to and forming part of thisinvention.

Briefly, in accordance with our invention, one form contemplates the useof a resistor comprising a number of serially-connected resistancesections with alternate lateral voltage taps passing through contactorsto two bus Wires which may be alternately or consecutively connected ina load circuit to place sections of the resistor serially in the loadcircuit. The invention is particularly but not exclusively associatedwith a switching means commonly referred to as a drum controller, whichmay be operated automatically as disclosed in Letters Patent 2,566,898,issued to l. W. Lichtenfels and H. G. Moore on September 4, 1951, andassigned to the same assignee as this invention.

In operation, after the current in the resistor has reached apredetermined minimum, a section of the resistor is shunted. When thecurrent again has decreased sufficiently, the shunted section isconnected between the two bus wires in parallel with another section.This process is continued until all of the parallel sections are againshunted, at which time the shunted sections are selectively placed inparallel with thenext consecutive section of the resistance.

In order that the invention may be clearly understood, reference nowwill be made by way of example to the accompanying drawings in which:

FIG. 1 is a schematic diagram illustrating one embodiment of theinvention as applied to electric traction motors.

FIG. 2 is a sequence chart showing two possible predetermined sequencesof operation of the controller, the total resistance of the resistorportion of the circuit and the resistance reduction between notches fora particular set of values of resistance sections, the contactors thatare closed by the controller, and the resistor sections that areutilized during each notch.

FIG. 3 shows diagrams of the resistance circuit shoW- ing thepredetermined arrangement of the main resistance sections in variousnotches of symmetric resistance shunting.

Referring to FIG. 1, the equipment shown illustrates contactors 1, 2, 3,4, 5, 6, 7, 8 and 9 for connecting a current limiting load resistorhaving serially connected resistance sections A, B, C, D, E, F, G and Hwith voltage taps 11, 12, 13, 14, 15, 16, 17 and 18 therebetween. Thisresistor is serially connectable with the variable impedance load shownas the electrical traction motors 19 and 20, and the series motor fieldwindings 21 and 22 through the contactor 1 and the bus wire 23. Thecontactor 2 connects the bus wire 24 and voltage tap 11 to the motors 19and 2t Contactors 1 and 2 are operable to connect the bus wires 23 and24 to point P. The contactors 3, 5, 7 and 9 each have one terminalconnected to bus 23 and the other terminal connected to voltage taps 12,14, 16 and 18 respectively. Similarly, contactor-s 4, 6 and 8 connectthe bus wire 24 to the voltage taps 13, and 17 respectively. All of thecontactors may be operated by a single cam or drum controller 25. In thepreferred arrangement the bus wires 23 and 24 are adapted to beconnected to separate alternate voltage taps in predeterminedarrangements as shown in FIG. 2 which illustrates the cam development ofthe controller 25.

In order that the motors 19 and may be reversible, their respectivefield windings 21 and 22 are connected by the reversing switch shownschematically at 26.

When accelerating the series D.C. traction motors 19 and 20, the poweris received by the pantograph 27 and passes through the starting orovercurrent switch 28 to the motors 19 and 2t) and the series fieldwindings 21 and 22. The current then passes through a portion of theresistor ABCDEFGH with one of the contactors 1 or 2 closed connectingbus wire 23 or 24 respectively in the load circuit, and to groundthrough the contactor or switch 31. In order that the resistor ABCDEFGHmay be placed in series with the traction motors 19 and 20 across thepower line for initial acceleration, or may be connected across themotors 19 and 20 for dynamic braking, or may be connected in parallelwith the field windings 21 and 22 to further weaken the field producedby the field windings to further accelerate the traction motors, thecontactor 31 is shown schematically as triplethrow switch which may, ofcourse, be operated by any well known means such as cams of thecontroller 25, manually, relays, etc.

Assuming switches 28 and 3-1 are closed to ground the system through themotors and the engineman is ready to accelerate the traction motors 19and 20 from standstill, the accelerating drum controller having thecontact sequence shown in FIG. 2 is moved to notch 1, which, as shown byFIG. 2, closes contactor 1 to serially connect the bus wire 23 andconnect the entire series resistor ABCDEFGH in the circuit (see FIG. 3,notch 1) between the varying impedance load of the motors 19 and 20 andthe ground return of the system. As the motors 19 and 20 accelerate,they develop a backvoltage which reduces the current through seriescircuit including the resistor ABCDEFGH. At some predetermined minimumcurrent, the drum controller is moved to notch 2, connecting bus wire 24to the motor circuit and increasing the current of the series circuit byshunting the resistance section A. This increase of current increasesthe torque of the motors 19 and 20 to further accelerate the tractionequipment.

Of course the controller may be automatically moved from one notch tothe next to close and open the contactors, but this is not a portion ofthis invention and will not be explained further. One control systemshowing automatic control of a motor operated cam controller is fullyexplained in the aforementioned Letters Patent 2,566,898.

For the purpose of explanation, we have assigned a set is reduced to6.77 ohms.

of specific values to the resistance sections A, B, C, D, E, F, G and H.For instance, the resistors sections A, B, C, D, E, F, G and H may havethe resistance of 2.05, 2.05, 1.15, 1.15, .68, .68, .55 and .51 ohmsrespectively.

Thus, when resistance section A is shunted by moving from notch 1 tonotch 2, the total resistance of 8.82 ohms It will be noted in FIG. 3that closing contactor 2 in notch 2 connects the base wire 24 to shuntcontactor 1 and eliminate current therethrough. From notch 2 to notch 3,the contactor 1 is opened without arcing damage because of this lack ofcurrent therethrough, and contactor 3 is then closed placing resistancesections A and B in parallel between the bus wires 23 and 24. Theparallel resistance sections A and B are serially connected to seriessections C, D, E, F, G and H as shown in notch 3 of FIG. 3. In moving tonotch 4, contactor 1 is closed and bus wire 23 shunts resistancesections A and B through the closed contactors 1 and 3 and eliminatesthe current through contactor 2. On proceeding to notch 5, contactor 2is opened and contactor 4 is closed to place sections A, B and C inparallel between bus wires 23 and 24. These parallel sections A, B and Care serially connected to series resistance sections D, E, F, G and Hthrough contactor 4 and voltage tap 13. Proceeding to notch 6 closescontactor 2 to shunt sections A, B and C through bus wire 24 andcontactor 4 to eliminate the current through the bus wire 23 and thecontactors 1 and 3 to allow these contactors to be opened withoutarcing.

Proceeding to notch 7 closes contactor 5 to connect bus wire 23 tovoltage tap 14 and to place resistance sections A and D in parallelbetween bus wires 23 and 24. In this notch the resistance sections B andC remain shunted by bus wire 24 which is connected at the voltage tap 11and through the contactor 4 to the voltage tap 13. The parallelcombination of resistance sections A and D is connected in series withresistance sections E, F, G and H through the contactor 5 which isconnected to the voltage tap 14. Proceeding to notch 8 closes contactor3 and places each of the resistance sections A, B, C and D in parallelbetween bus wires 23 and 24 with the parallel combination connected inseries with series sections E, F, G and H through contactor 5.Proceeding to notch 9 closes the contactor 1 to shunt resistancesections A, B, C, D and contactors 2, 3 and 4 through bus wire 23 andthe contactor 5, so that the contactors 2, 3 and 4 may be opened withoutthe problem of arcing.

In the predetermined symmetric sequence shown in FIG. 2, notches 1through 30, it should be noted that no contactor is opened unless bothbus wires 23 and 24 are connected through contactors 1 and 2respectively, so that the opening contactors are shunted to preventarcing damage to the contactors.

The symmetric sequence shown in notches 1 through 30 of FIGURES 2 and 3places resistance section A in the circuit 77% of the time. Theresistance sections B, C, D, E, F, G and H are in the circuit 70, 67,67, 70, 67, 87 and of the time, respectively. This utilization of thevarious resistance sections for such a large percentage of the time usesmuch less resistor mass than a larger number of sections for a smallerpercentage of the time. Thus, we are able to considerably reduce thebulk of the resistor, and also reduce the number of resistance sections,while still getting the necessary number of steps for the particularapplication.

This system of resistance shunting results in 30 resistance notches witheight sections of resistor shown in FIG- URES 1 and 3. If a greaternumber of notches is required for a particular application, this systemwill result in 38 notches with 9 sections, 47 notches with 10 sections,57 notches with 11 sections, 68 notches with 12 sections, etc. This ispossible, following a predetermined symmetric type progression, evenwith the limitation of never opening a contactor that is carryingcurrent, i.e., first placing resistor A in parallel with the lastsection to be connected,

then placing A and B in parallel, then A, B and C, etc.

The resistance shunting system shown in FIG. 2 as notches 11a through2511 is still subject to the limitation of never opening a currentcarrying contactor. However, notches 11, 16, 19, 21 and 26 have beenomitted to shorten the sequence and make the steps used more nearlygeometric.

Other combinations of the resistance sections are of course possible by,for instance, closing contactors in the combination 1, 6, 7, 8 whichwould give an impedance of .79 ohm, or 2, 7, '8, 9 which would give animpedance of .264 ohm. However, when advancing from these nonsymmetricnotches in a predetermined arrangement to the next lower resistanceposition, it becomes more and more difficult to arrange the contactorsin a progression whereeach contactor is opened after a notch where ithas been shunted.

Of course the basic circuit as shown in FIG. 1 can be used with switcheshaving provision for opening a current carrying contactor. The additionof arc arresters, increase of the air gap of the contactors, etc. mayincrease the current capacity of the contactors. Additional combinationsare feasible, and for some applications of the circuit such switchingsequences may be desirable. However, this increase of possible steps maybe made uneconomical by the necessity of arc arrestors, increase in contactor air gap, etc. In heavy current applications this practice islikely to necessitate increased maintenance costs.

In summary, the double bus wires 23 and 24 in this resistance switchingnetwork provide for a convenient means of reaching a corner point orpoint from which parallel connections may be made as soon as one sectionof the resistor has been shunted. Thus, when resistance sections areshunted, these resistance sections may be placed in parallel with aresistance section of the unshunted resistor to partially shunt it andreduce the total resistance geometrically, as from notch 2 to notch 3,or notch 4 to notch 5, etc. In this network, at any time contactors 1and 2 are closed, as in notch 2, 4, 6, 9, 13, 18, 24 or 31, all of theshunted contactors between the contactor last closed and the power linemay be opened without the danger of arcing because of the shuntingarrangement provided. With this predetermined switching arrangement,using the symmetrical approach as shown in notches 1 through 31, it issimple matter to calculate the impedance of the resistance shuntingsystem. It is apparent that this arrangement uses fewer contactors toprovide more steps than previous systems, because of the fact thatcorner points are utilized more quickly than in previous systems. Also,it is readily apparent that every section of the resistor element isused a larger percentage of the time than was practicable in pastresistance shunting schemes.

While we have shown and described a particular embodiment of thisinvention, further modifications and improvements will occur to thoseskilled in the art. We desire it understood, therefore, that thisinvention is not limited to the forms shown and we intend by theappended claims to cover all modifications which do not depart from thetrue spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In a controller circuit for a series electric motor, a plurality ofserially connected resistance sections, a voltage tap between each ofthe sections, a bus wire connected to one end of the resistancesections, a second bus wire connected to the voltage tap next adjacent,contactors for selectively connecting the r maining voltage taps to thefirst and second bus wires alternately and for connecting the bus wiresin circuit with the motor, and controller means for sequentiallyoperating the contactors to connect the resistance sections initially inseries by connecting the first bus wire in circuit with the motor andthereafter to shunt one section by connecting the second wire to themotor circuit at the same point of connection as the first bus wire, toconnect the shunted section in parallel with a remaining section bydisconnecting the first bus wire from the point and connecting the firstbus wire to one of the voltage taps, to shunt the parallel sections byconnecting the first bus wire to the same point, and then selectively toconnect the shunted sections in parallel with a remaining section bydisconnecting the second bus wire from the motor and selectivelyconnecting the second bus wire to one of the voltage taps, thisoperation being repeated until substantially all of the sections areconnected in parallel with one another in the motor circuit.

2. In a controller circuit for a direct current electric motor providedwith a series field winding, a resistance switching network comprising aplurality of serially connected resistance sections, a voltage tapbetween each of the resistance sections, a bus wire connected to one endf the resistance sections, a second bus wire connected to a firstvoltage tap next adjacent to the one end, contactors for selectivelyconnecting the remaining voltage taps alternately to the first and thesecond bus wires and for connecting the bus wires in circuit with themotor, a cam operated controller operatively connected to the contactorsfor regulating the impedance of the network by sequentially operatingthe contactors to connect the resistance sections initially in series byconnecting the first bus wire in circuit with the motor and thereafterto shunt one section by connecting the second wire to the motor circuitat the same connection point as the first bus wire, to connect theshunted section in parallel with the adjacent sections remaining bydisconnecting the first bus wire from the point and connecting the firstbus wire to a second voltage tap adjacent to the first tap, to shunt theparallel sections by connecting the first bus wire to the sameconnection point as the second bus wire, and then selectively to connectthe shunted sections in parallel with a remaining section bydisconnecting the second bus wire from the point and selectivelyconnecting the second bus wire to a third voltage tap adjacent to thesecond voltage tap, this operation being repeated until substantiallyall of the sections are connected in parallel with one another in themotor circuit, and other contactor for selectively connecting theresistor sections in circuit with the motor for dynamic braking orfurther field weakening.

3. A resistance switching network for controlling the value of currentin an electric circuit comprising, a plurality of serially connectedresistance segments having taps therebetween, first and second bus wiresconnectable at one end to a common point in the electric circuit, saidfirst bus wire being directly connected to one end of said segments,said one end being connectable through first switching means to saidcommon point, said second bus wire being directly connected to the tapnext adjacent said one end, second switching means for connecting saidtap next adjacent to said common point, the remainder of said taps beingalternately connected through other switching means to said first andsecond bus wires, means for connecting the second end of said seriallyarranged segments to complete the electric circuit, and controller meansto selectively operate said first and second switching means and saidother switching means to progressively vary the effective resistance ofthe network.

4. A resistance switching network for controlling the value of currentin an electric circuit comprising, a plurality of serially connectedresistance segments having taps therebetween, first and second bus wiresconnectable at one end to a common point in the electric circuit, saidfirst bus wire being directly connected to one end of said segments,said one end being connectable through first switching means to saidcommon point, said second bus wire being directly connected to the tapnext adjacent said one end, second switching means for connecting saidtap next adjacent to said common point, the remainder of said taps beingalternately connected through other switching means to said first andsecond bus wires, means for connecting the second end of said seriallyarranged segments to complete the electric circuit, and controller meansto selectively operate said first and second switching means and saidother switching means to progressively vary the effective resistance ofthe network, said other switching means made operable to interruptcurrent fiow only when both said first and said second switching meansare closed.

5. A resistance switching network for controlling the value of currentin an electric circuit which includes at least one direct current motorhaving a series field winding, comprising, a plurality of seriallyconnected resistance segments having taps therebetweemfirst and secondbus wires connectable at one end to a common point in the electriccircuit, said first bus Wire being directly connected to one end of saidsegments, said one end being connectable through first switching meansto said common point, said second bus wire being directly connected tothe tap next adjacent said one end, second switching means forconnecting said tap next adjacent to said common point, the remainder ofsaid taps being alternately connected through other switching means tosaid first and second bus wires, means for connecting the second end ofsaid serially arranged segments to complete the electric circuit, all ofsaid other switching means being directly connected through said buswires to said first or said second switching means, and means toselectively operate all of said switching means to progressively varythe eifective resistance of the network in a predetermined manner.

6. A resistance switching network for controlling the value of currentin an electric circuit which includes at least one direct current motorhaving a series field winding, comprising, a plurality of seriallyconnected resistance segments having taps therebetween, first and secondbus wires connectable at one end to a common point in the electriccircuit, said first bus wire being directly connected to one end of saidsegments, said one end being connectable through first switching meansto said common point, said second bus wire being directly connected tothe tap next adjacent to said one end, second said first and second buswires, means for connecting the second end of said serially arrangedsegments to complete the electric circuit, all of said other switchingmeans being directly connected through said bus wires to said first orsaid second switching means, and means to selectively operate all ofsaid switching means to progressively vary the efiective resistance ofthe network in a predetermined manner, said other switching means madeoperable to interrupt current flow only when both said first and saidsecond switching means are closed.

7. A resistance switching network for controlling the value of currentin an electric circuit comprising, a plurality of serially connectedresistance segments having taps therebetween, first and second bus wiresconnectable at one end to a common point in the electric circuit, saidfirst bus wire being directly connected to one end of said segments,said one end being connectable through first switching means to saidcommon point, said second bus wire being directly connected to the tapnext adjacent said one end, second switching means for connecting saidtap next adjacent to said common point, the remainder of said taps beingalternately connected through other switching means to said first andsecond bus wires, means for connecting the second end of said seriallyarranged segments to complete the electric circuit, all of said otherswitching means being directly connected through said bus wires to saidfirst or said second switching means, and means to selectively operateall of said switching means to vary the effective resistance of thenetwork in a predetermined manner.

References Cited in the file of this patent UNITED STATES PATENTS2,131,588 Gray Sept. 27, 1938

